Addition of N-Heterocyclic Carbene Catalyst to Aryl Esters Induces Remote C-Si Bond Activation and Benzylic Carbon Functionalization

Article abstract of DOI:10.1021/acs.orglett.7b03544

Through the incorporation of a silicon atom to an aryl carboxylic ester substrate, the resulting C-Si bond can be activated via the addition of a carbene catalyst on a remote site. This strategy allows for efficient functionalization of the benzylic sp³-carbons of aryl carboxylic esters.

Full text of DOI:10.1021/acs.orglett.7b03544

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Addition of N‑Heterocyclic Carbene Catalyst to Aryl Esters Induces
Remote C−Si Bond Activation and Benzylic Carbon
Functionalization
Hongling Wang,†,§ Xingkuan Chen,‡,§ Yongjia Li,‡ Jilan Wang,† Shuquan Wu,† Wei Xue,†
Song Yang,*,† and Yonggui Robin Chi*,†,‡
†Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key Laboratory of Green Pesticide and Agricultural
Bioengineering, Ministry of Education, Guizhou University, Huaxi District, Guiyang 550025, People’s Republic of China
‡Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, Singapore 637371
S
* Supporting Information
ABSTRACT: Through the incorporation of a silicon atom to an aryl carboxylic ester
substrate, the resulting C−Si bond can be activated via the addition of a carbene
catalyst on a remote site. This strategy allows for efficient functionalization of the
benzylic sp3-carbons of aryl carboxylic esters.
he utilization of silicon atom in organic synthesis has
T
enjoyed enormous success.1 For instance, carbon−silicon
bond is weaker than carbon−carbon bond, and the silicon atom
has strong affinity with fluoride.2 These properties have allowed
chemists to modulate the reactivities of other atoms by
incorporating silicon to the molecules. N-Heterocyclic carbenes
(abbreviated as NHCs or carbenes)3 have been proven to be a
powerful class of organic catalysts that can often enable unique
activation and reaction modes. However, it is still a challenge to
bring the benefits of carbene catalysis for the functionalization of
aromatic sp2-carbons and the attached sp3-carbons, such as
benzylic carbons.4 For example, our long-term efforts to activate
the sp2 and sp3-carbons of the simple aryl aldehydes (such as 2-
methylbenzaldehyde A) remain unsuccessful until date (Figure
1a). To overcome this problem, we found that by incorporating
an electronegative heteroatom (such as N) to the aromatic
scaffold and using indole-derived aryl aldehydes (B) as the
substrates, the nearby sp3-carbon of methyl unit can be
functionalized under oxidative NHC catalytic conditions.5
Recently, the Glorius and Rovis groups found that the
introduction of an electronegative Br atom to an aryl aldehyde
(C) can lead to NHC-mediated functionalization of its benzylic
carbon.6a,b It is noted that when the 2-(bromomethyl)-
benzaldehyde was used, the reaction typically gave moderate
yields. Side reactions resulting from the electronegative property
of Br atom (as a leaving group) led to unproductive
consumption of both the carbene catalyst and aldehyde
substrate.6a Conventional approaches in using strong bases for
deprotonation of benzylic CH7 have very limited controls over
chemo- and stereoselectivities. Reported photoinitiation
approaches8 do not provide handles for chiral inductions.
We reasoned that when electropositive atoms (such as
silicon) are properly incorporated to the aryl aldehyde/ester9
Figure 1. NHC-catalyzed activation and reaction modes of aryl
aldehydes and esters.
substrates, different reactivity patterns shall become possible
(Figure 1b). Here we report that by using 2-[(trimethylsilyl)-
Received: November 15, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03544
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
methyl]benzoate with a Si atom attached to the benzylic carbon
as the substrate, the addition of a carbene catalyst to the ester can
activate the remote C−Si bond (intermediate I). An F− anion
attacks the Si atom of I and leads to a breakage of the C−Si
bond2e,f that is already weakened by the azolium moiety from the
NHC catalyst. This step generates intermediate ο-quinodi-
methane (ο-QDM) II.10 This intermediate (II) then undergoes
formal [4 + 2] reaction with reactive ketone to form a δ-lactone
product.11
Our catalytic reactions based on C−Si bond modulation are
relatively clean, and products are obtained with good to excellent
yields. Notably, the use of Si atom to facilitate NHC-mediated
activation is mostly unexplored. To the best of our knowledge,
the most relevant study in this direction is Scheidt’s elegant use
of acylsilanes as aldehyde equivalents to generate Breslow
intermediates.12
reported in several studies.15 Replacing the N-phenyl substituent
of the NHC catalysts with a mesityl unit led to the formation of
3a with good yield (entries 3 and 6). The superior effectiveness
of N-mesityl substituted triazolium NHC catalysts (over the
corresponding N-phenyl analogs) in carboxylic ester activation
reactions has been observed in several of our previous
studies.9d−g We next found that N−C6F5 substituted catalysts
performed even better yields (entries 3 and 7). When catalyst
C616 was used, product 3a was formed in 88% yield (entry 7).
The switch of Cs2CO3 to K2CO3 as the base led to a small, while
consistent, improvement of the reaction yield (entry 8). The use
of organic bases (such as DMAP, DABCO, and DIEA) could
also lead to efficient reactions (entries 9−11). The presence of
CsF is critical (for the generation of key intermediate II as
illustrated in Figure 1b), and no product could be obtained
under various conditions when CsF was not added (entry 12).
Our efforts for enantioselective reactions using chiral NHC
catalysts did not give satisfied results at this point (see SI). It
appears under this type of reaction mode (e.g., with intermediate
II involved in the key step, Figure 1b), the current chiral NHC
catalysts were ineffective for enantioselective inductions, as
observed in several related reactions.6a,b
Key results of condition optimization, by using 2-
[(trimethylsilyl)-methyl]benzoate (1a) and trifluoromethyl
ketone (2a) as the model substrates, are summarized in Table
1. The reactions were performed in THF as the solvent at room
a
Table 1. Optimization of Conditions
We next used the optimal condition (Table 1, entry 8) to
examine the scope of the reactions between 2-[(trimethylsilyl)-
methyl]benzoate (1) and trifluoromethyl ketones (2) (Scheme
1). Installation of different substituents to the phenyl ring of the
trifluoroketones were all tolerated, giving the desired lactone
compounds in 70−94% yields (3a−e). Replacing the phenyl
unit of 2 to heteroaryl substituent did not affect the reaction
outcomes (3f). When the aryl group of trifluoroketone was
replaced with alkyl substituent, the lactone product (3g) was
a
b
Scheme 1. Examples of Esters and Trifluoromethyl Ketones
entry
condition
no NHC
yield
c
1
nr
2
3
C1
C2
trace
77
4
C3
84
5
6
C4
C5
trace
62
7
C6
88
8
9
10
11
12
C6, K2CO3
C6, DMAP
C6, DABCO
C6, DIEA
as entry 8, no CsF
94
67
76
79
nr
a
Reaction conditions: 1a (0.12 mmol), 2a (0.10 mmol), NHC (0.02
mmol), base (0.1 mmol), 4 Å MS (100 mg), THF (2.0 mL), rt, 36 h.
b
Yield are isolated yield after purification by column chromatography;
c
isolated yield based on 2a. nr = no reaction. DIEA = N,N-
diisopropylethylamine. DMAP = 4-dimethylaminopyridine. DABCO
= 1,4-diazabicyclo[2.2.2]octane.
temperature with Cs2CO3 as the base and CsF as an F− source to
break the Si−C bond. No product (3a) was observed in the
absence of NHCs, indicating that the formation of azolium ester
intermediate I (Figure 1b) is necessary to activate the remote
C−Si bond for further reactions. The ester substrate (1a)
remained unreacted without the presence of NHC. Triazolium-
based NHC precatalysts were then examined. Catalysts with an
N-phenyl substituent (C113 and C414) were ineffective (entries
2 and 5) likely due to the ortho-substitution effect;15c in both
reactions the substrates were recovered. Similar observations
using the sterically small N-phenyl substituent NHCs were
a
Reaction conditions: 1 (0.12 mmol), 2 (0.10 mmol), C6 (0.02
mmol), K2CO3 (0.1 mmol), 4 Å MS (100 mg), THF (2.0 mL), rt, 36
h. Yields (after SiO2 column chromatography purification) based on
the ketones 2.
B
DOI: 10.1021/acs.orglett.7b03544
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
also obtained with good yield (75%). Different substituents and
substitution patterns on the aryl carboxylic ester substrates (1)
were also tolerated (3h−o). It should be noted that in most of
the examples the reaction gave lactone products with over 80%
yields. In contrast, with previous approaches using 2-
(bromomethyl)-benzaldehyde substrates, most of the reactions
give low to moderate yields.
The products of our catalytic reactions contain a lactone
moiety that is widely found in natural products and other
bioactive molecules. Our laboratories are interested in the
antiviral and antibacterial activities of these compounds for
potential agricultural use.17 We then evaluated the in vitro
bioactivities of our products against X. oryzae by the
turbidimeter test (Table 2). The commercially available and
α-Ketoesters were also effective electrophiles for reactions
with 2-[(trimethylsilyl)-methyl]benzoate 1a (Scheme 2). In
Table 2. Preliminary Evaluations on the Antibacterial
Activities of Our Products
a
Scheme 2. Ketoesters and Isatins as Electrophilic Substrates
To React with the Aryl Ester 1a
X. oryzoe pv. oryzae inhibition rate [%]
a
product
200 μg/mL
100 μg/mL
3c
5h
7e
52 1.16
88 0.41
89 0.52
70 0.85
0
41 1.71
48 1.50
53 1.24
54 1.11
0
positive control
negative control
a
Data is the average of three replicates. Commercial bacteriocide
bismerthiazol was used as the positive control, and DMSO was used
as the negative control.
commonly applied bacteriocide bismerthiazol was used as the
positive control, and dimethyl sulfoxide (DMSO) was used as
the negative control. A number of our compounds showed
promising antibacterial activities. For example, at concentrations
of 200 and 100 μg/mL, compounds 3c, 5h, and 7e showed
inhibitory rates comparable to the commercial bacteriocide
bismerthiazol against X. oryzae.18
In summary, we have developed a carbene-catalyzed approach
for the functionalization of benzylic carbons of aryl carboxylic
esters. The approach relies on the incorporation of a C−Si bond
that can be further activated through the addition of a carbene
catalyst to the ester moiety of the substrate. Several types of
activated ketones could react effectively as electrophiles.
Products of our catalytic reactions showed promising anti-
bacterial activities against X.oryzae. We believe this study shall
inspire new exploration on carbene-catalyzed activations that go
beyond carbon−carbon bonds.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03544.
a
Reaction conditions: 1a (0.12 mmol), 4 or 6 (0.10 mmol), NHC
Experimental procedures, characterization data, and
calculation details (PDF)
(0.02 mmol), base (0.1 mmol), 4 Å MS (100 mg), THF (2.0 mL), rt,
36 h. Yields (after SiO2 column chromatography purification) based
on the electrophiles 4 or 6.
AUTHOR INFORMATION
Corresponding Authors
■
these reactions, the optimal condition used for trifluoromethyl
ketone substrates (Table 1, entry 8) gave 5a with 68% yield. The
switch of the NHC catalyst from C6 to C313 led to a consistently
higher yield (e.g., 75% yield of 5a). We finally found that by
using C3 as the NHC precatalyst and DABCO as the base, the
reaction could give product 5a in excellent yield (91%). Under
this slightly modified condition, various ketoester substrates
were tolerated, and most of the reactions gave excellent yields
(5a−5h). We also studied isatins as ketone substrates. Cs2CO3
was found as an optimal base, and these isatin substrates
examined here led to spirocyclic lactone products with excellent
yields (7a−7h, 80−94% yields).
*E-mail: robinchi@ntu.edu.sg.
*E-mail: yangsdqj@126.com.
ORCID
Xingkuan Chen: 0000-0002-1974-1437
Yonggui Robin Chi: 0000-0003-0573-257X
Author Contributions
§These authors contributed equally.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.7b03544
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
8182. (c) Candish, L.; Lupton, D. W. Chem. Sci. 2012, 3, 380. (d) Hao,
L.; Du, Y.; Lv, H.; Chen, X.; Jiang, H.; Shao, Y.; Chi, Y. R. Org. Lett.
2012, 14, 2154. (e) Fu, Z.; Xu, J.; Zhu, T.; Leong, W. W. Y.; Chi, Y. R.
Nat. Chem. 2013, 5, 835. (f) Hao, L.; Chan, W. C.; Ganguly, R.; Chi, Y.
R. Synlett 2013, 24, 1197. (g) Xu, J.; Jin, Z.; Chi, Y. R. Org. Lett. 2013,
15, 5028. (h) Cheng, J.; Huang, Z.; Chi, Y. R. Angew. Chem., Int. Ed.
2013, 52, 8592. (i) Hao, L.; Chen, S.; Xu, J.; Tiwari, B.; Fu, Z.; Li, T.;
Lim, J.; Chi, Y. R. Org. Lett. 2013, 15, 4956. (j) Hao, L.; Chen, X.; Chen,
S.; Jiang, K.; Torres, J.; Chi, Y. R. Org. Chem. Front. 2014, 1, 148. (k) Fu,
Z.; Wu, X.; Chi, Y. R. Org. Chem. Front. 2016, 3, 145. (l) Chen, X.; Fong,
J. Z. M.; Xu, J.; Mou, C.; Lu, Y.; Yang, S.; Song, B.-A.; Chi, Y. R. J. Am.
Chem. Soc. 2016, 138, 7212.
(10) (a) Cava, M. P.; Napier, D. R. J. Am. Chem. Soc. 1957, 79, 1701.
(b) Charlton, J. L.; Alauddin, M. M. Tetrahedron 1987, 43, 2873.
(c) Segura, J. L.; Martin, N. Chem. Rev. 1999, 99, 3199. (d) Yoshida, H.;
Ohshita, J.; Kunai, A. Bull. Chem. Soc. Jpn. 2010, 83, 199. (e) Collier, S.
J.; Storr, R. C. Prog. Heterocycl. Chem. 1998, 10, 25.
(11) (a) Shen, L.-T.; Shao, P.-L.; Ye, S. Adv. Synth. Catal. 2011, 353,
1943. (b) Jia, W.-Q.; Zhang, H.-M.; Zhang, C.-L.; Gao, Z.-H.; Ye, S.
Org. Chem. Front. 2016, 3, 77. (c) Mo, J.; Chen, X.; Chi, Y. R. J. Am.
Chem. Soc. 2012, 134, 8810. (d) Wang, M.; Huang, Z.; Xu, J.; Chi, Y. R.
J. Am. Chem. Soc. 2014, 136, 1214. (e) Li, B.-S.; Wang, Y.; Jin, Z.; Zheng,
P.; Ganguly, R.; Chi, Y. R. Nat. Commun. 2015, 6, 6207. (f) Chen, X.-Y.;
Xia, F.; Cheng, J.-T.; Ye, S. Angew. Chem., Int. Ed. 2013, 52, 10644.
(g) Cheng, J.-T.; Chen, X.-Y.; Gao, Z.-H.; Ye, S. Eur. J. Org. Chem. 2015,
2015, 1047.
(12) (a) Mattson, A. E.; Bharadwaj, A. R.; Scheidt, K. A. J. Am. Chem.
Soc. 2004, 126, 2314. (b) Bharadwaj, A. R.; Scheidt, K. A. Org. Lett.
2004, 6, 2465. (c) Mattson, A. E.; Zuhl, A. M.; Reynolds, T. E.; Scheidt,
K. A. J. Am. Chem. Soc. 2006, 128, 4932.
ACKNOWLEDGMENTS
■
We acknowledge financial support by the National Natural
Science Foundation of China (No. 21472028), National Key
Technologies R&D Program (No. 2014BAD23B01), “Thou-
sand Talent Plan”, The 10 Talent Plan (Shicengci) of Guizhou
Province, Guizhou Province Returned Oversea Student Science
and Technology Activity Program, Guizhou University (China)
and Singapore National Research Foundation (NRF-
NRFI2016-06), the Ministry of Education of Singapore
(MOE2013-T2-2-003; MOE2016-T2-1-032; RG108/16),
A*STAR Individual Research Grant (A1783c0008), and
Nanyang Technological University
REFERENCES
■
(1) (a) Brook, A. G.; Mauris, J. J. Am. Chem. Soc. 1957, 79, 971.
(b) Brook, A. G. Acc. Chem. Res. 1974, 7, 77. (c) Yamamoto, K.; Suzuki,
S.; Tsuji, J. Tetrahedron Lett. 1980, 21, 1653. (d) Ricci, A.;
Degl'Innocenti, A.; Chimichi, S.; Fiorenza, M.; Rossini, G. J. Org.
Chem. 1985, 50, 130. (e) Page, P. C. B.; Klair, S.; Rosenthal, S. Chem.
Soc. Rev. 1990, 19, 147. (f) Higashiya, S.; Chung, W. J.; Lim, D. S.; Ngo,
S. C.; Kelly, W. H.; Toscano, P. J.; Welch, J. T. J. Org. Chem. 2004, 69,
6323. (g) Srihari, P.; Yaragorla, S. R.; Basu, D.; Chandrasekhar, S.
Synthesis 2006, 16, 2646. (h) Beeson, T. D.; Mastracchio, A.; Hong, J.-
B.; Ashton, K.; MacMillan, D. W. C. Science 2007, 316, 582.
(2) (a) Schinzer, D.; Heathcock, C. H. Tetrahedron Lett. 1981, 22,
1881. (b) Ricci, A.; Degl'Innocenti, A.; Fiorenza, M.; Taddei, M.;
Spartera, M. A.; Walton, D. R. M. Tetrahedron Lett. 1982, 23, 577.
(c) Padwa, A.; Dent, W.; Nimmesgern, H.; Venkatramanan, M. K.;
Wong, G. S. K. Chem. Ber. 1986, 119, 813. (d) Padwa, A.; Dent, W. J.
Org. Chem. 1987, 52, 235. (e) Ryan, J. H.; Spiccia, N.; Wong, L. S.-M.;
Holmes, A. B. Aust. J. Chem. 2007, 60, 898. (f) Das, M.; O’Shea, D. F.
Tetrahedron 2013, 69, 6448.
(13) For pioneering report on NHC catalyst C1 and C3, see: Kerr, M.
S.; Read de Alaniz, J.; Rovis, T. J. Org. Chem. 2005, 70, 5725.
(14) For pioneering report on NHC catalyst C4, see: Massey, R. S.;
Collett, C. J.; Lindsay, A. G.; Smith, A. D.; O’Donoghue, A. C. J. Am.
Chem. Soc. 2012, 134, 20421.
(3) For selected reviews on NHC catalysis, see: (a) Grossmann, A.;
Enders, D. Angew. Chem., Int. Ed. 2012, 51, 314. (b) Mahatthananchai,
J.; Bode, J. W. Chem. Sci. 2012, 3, 192. (c) Cohen, D. T.; Scheidt, K. A.
Chem. Sci. 2012, 3, 53. (d) Bugaut, X.; Glorius, F. Chem. Soc. Rev. 2012,
41, 3511. (e) Ryan, S. J.; Candish, L.; Lupton, D. W. Chem. Soc. Rev.
2013, 42, 4906. (f) Hopkinson, M. N.; Richter, C.; Schedler, M.;
Glorius, F. Nature 2014, 510, 485. (g) Mahatthananchai, J.; Bode, J. W.
Acc. Chem. Res. 2014, 47, 696. (h) Flanigan, D. M.; Romanov-
Michailidis, F.; White, N. A.; Rovis, T. Chem. Rev. 2015, 115, 9307.
(i) Menon, R. S.; Biju, A. T.; Nair, V. Beilstein J. Org. Chem. 2016, 12,
444. (j) Wang, M. H.; Scheidt, K. A. Angew. Chem., Int. Ed. 2016, 55,
14912. (k) Zhang, C.; Hooper, J. F.; Lupton, D. W. ACS Catal. 2017, 7,
2583.
(4) (a) Charlton, J. L.; Alauddin, M. M. Tetrahedron 1987, 43, 2873.
(b) Segura, J. L.; Martin, N. Chem. Rev. 1999, 99, 3199.
(5) (a) Chen, X.; Yang, S.; Song, B.-A.; Chi, Y. R. Angew. Chem., Int.
Ed. 2013, 52, 11134. (b) Xu, J.; Yuan, S.; Miao, M. Org. Lett. 2016, 18,
3822. (c) Chen, X.; Wang, H.; Doitomi, K.; Ooi, C.; Zheng, P.; Liu, W.;
Guo, H.; Yang, S.; Song, B.-A.; Hirao, H.; Chi, Y. R. Nat. Commun.
2017, 8, 15598.
(15) (a) Mahatthananchai, J.; Bode, J. W. Chem. Sci. 2012, 3, 192.
(b) Collett, C. J.; Massey, R. S.; Maguire, O. R.; Batsanov, A. S.;
O’Donoghue, A. C.; Smith, A. D. Chem. Sci. 2013, 4, 1514. (c) Collett,
C. J.; Massey, R. S.; Taylor, J. E.; Maguire, O. R.; O’Donoghue, A. C.;
Smith, A. D. Angew. Chem., Int. Ed. 2015, 54, 6887.
(16) For pioneering report on NHC catalyst C6, see: Kuwano, S.;
Harada, S.; Oriez, R.; Yamada, K. Chem. Commun. 2012, 48, 145.
(17) (a) Namitharan, K.; Zhu, T.; Cheng, J.; Zheng, P.; Li, X.; Yang,
S.; Song, B.-A.; Chi, Y. R. Nat. Commun. 2014, 5, 3982. (b) Zheng, P.;
Cheng, J.; Su, S.; Jin, Z.; Wang, Y.; Yang, S.; Jin, L.; Song, B.-A.; Chi, Y.
R. Chem. - Eur. J. 2015, 21, 9984. (c) Chen, X.; Wang, H.; Doitomi, K.;
Ooi, C.; Zheng, P.; Liu, W.; Guo, H.; Yang, S.; Song, B.-A.; Hirao, H.;
Chi, Y. R. Nat. Commun. 2017, 8, 15598.
(18) (a) Bos, L. Crop Prot. 1982, 1, 263. (b) Lamberth, C.; Jeanmart,
S.; Luksch, T.; Plant, A. Science 2013, 341, 742. (c) Wang, X.; Li, P.; Li,
Z.; Yin, J.; He, M.; Xue, W.; Chen, Z.; Song, B.-A. J. Agric. Food Chem.
2013, 61, 9575.
(6) (a) Janssen- Muller, D.; Singha, S.; Olyschlager, T.; Daniliuc, C.
̈
̈
G.; Glorius, F. Org. Lett. 2016, 18, 4444. (b) Chen, D.-F.; Rovis, T.
Synthesis 2017, 49, 293.
(7) (a) Masatoshi, Y.; Kuniko, H.; Masao, I.; Hidetoshi, O.; Kenji, T. J.
Med. Chem. 1981, 24, 194. (b) Fu, P. P.; Unruh, L.; Miller, D. W.;
Huang, L. W.; Yang, D. T. C. J. Org. Chem. 1985, 50, 1259. (c) Wu, J.-L.;
Du, B.-X.; Zhang, Y.-C.; He, Y.-Y.; Wang, J.-Y.; Wu, P.; Shi, F. Adv.
Synth. Catal. 2016, 358, 2777. (d) Cornaggia, C.; Gundala, S.; Manoni,
F.; Gopalasetty, N.; Connon, S. J. Org. Biomol. Chem. 2016, 14, 3040.
(e) Duan, J.; Cheng, Y.; Cheng, J.; Li, R.; Li, P. Chem. - Eur. J. 2017, 23,
519.
(8) Takaki, K.; Fujii, T.; Yonemitsu, H.; Fujiwara, M.; Komeyama, K.;
Yoshida, H. Tetrahedron Lett. 2012, 53, 3974.
(9) (a) Ryan, S. J.; Candish, L.; Lupton, D. W. J. Am. Chem. Soc. 2009,
131, 14176. (b) Candish, L.; Lupton, D. W. Org. Biomol. Chem. 2011, 9,
D
DOI: 10.1021/acs.orglett.7b03544
Org. Lett. XXXX, XXX, XXX−XXX